Logic vs. transistor driver

2ms/div
2V/div
CH1: Row 1 UDN2981A
CH2: Row 8 UDN2981A

PWM cycle of the rgb led matrix. The interrupt code runs when the signal wiggles. Each little dip scans the whole matrix for one brightness step. 32 scans make up the whole PWM cycle.

You can also observe an interesting effect here. The color that was displayed on the matrix has a hue value of 200, which translates to (0.000,0.666,1.000) in RGB. To set the status bit for a color to "on", the code nees to run 1 command, to leave it "off" nothing has to be processed. Therefore the code finishes a bit earlier. In the photo you'll see a slight difference near 2/3 of the wiggles. The period changes slightly from about 6 to about 7 wiggles per div. In this case you can guess the duty cycle of 0.66 for one color. It only works, as for red it is always 0 and for blue always 1, so only green changes at some point of time.